Thus, the object of the invention is to propose a sensor that generates the least possible turbulence when introduced in the site of application and that has a high mechanical stability, and also a method for manufacturing such a sensor.
This object is achieved by a sensor and a method.
In terms of the sensor, the object is achieved according to the invention by the fact that the sensor for determining at least one process variable of a medium comprises at least:                a metallic substrate that has a recess at least in a first region; and        a first dielectric layer that is arranged on the metallic substrate at least in the first region, wherein the first dielectric layer forms a membrane; and        at least one heating structure that is arranged in the first region on the first dielectric layer formed as a membrane, wherein the heating structure heats the medium; and        at least one temperature sensor element, which is associated with the first region, arranged on the first dielectric layer at a distance from the heating structure and detects the temperature of the medium heated at the heating structure; and        at least one protective layer that covers at least the at least one heating structure and the at least one temperature sensor element.        
By the use of a metallic substrate, which acts as a support and can be designed very thin, preferably with a thickness in the range of 50-250 microns (micrometers), but more preferably a thickness less than 200 microns, lower influence on the surroundings, for example, vortex formation, can be achieved by the reduction of the overall height of the sensor when it is being introduced in the designated site of application, so as to obtain a more precise or more accurate measurement result using the sensor. For example, if it is a flow sensor, it can be used in a flow channel without causing substantial turbulence. In addition to having the least influence on the surroundings, it is also desirable to obtain a fast and sensitive sensor, in particular, also in case of a flow sensor. This requires, among others, a thermal insulation of the heating structure from the temperature sensor element and a reduction in thermal mass. In order to reduce the thermal mass, the sensitive structures, namely the at least one heating structure and preferably also the at least one temperature sensor element are formed on a membrane.
To this end, the metallic substrate is typically structured by means of industrial grade etching in such a manner that it comprises a recess by means of which the first dielectric layer forms a membrane.
In the context of this invention, the area above the recess introduced in the metallic substrate is essentially considered to be the first region, wherein the first region relates, in particular, to the membrane and the layers and/or structures that are arranged above the membrane.
Furthermore, in the context of this invention, association with the first region refers to the fact that the respective element may be located within or partly within or outside this region, but in the event that an element is arranged only partially inside or outside the first region, this element interacts with at least one element, which is formed within the first region, via the medium.
According to the invention, a metal is used as the substrate, so as to achieve the reduction of the overall height of the sensor, since metals offer the advantage that they have a high mechanical stability even at very low strengths or thicknesses and are also suitable for use in industrial etching processes for their structuring. Furthermore, metallic substrates may be coated with materials that are not affected by the process for structuring the metallic substrate, wherein the materials applied as a coating on the metallic substrate may in turn be provided with functional metallic structures. Compared to the thermal flow sensors known from the prior art, the use of a metal substrate offers the advantage that metal allows structuring as opposed to ceramic substrates.
A first dielectric layer is deposited on the metallic substrate to prevent an electrical connection between the at least one heating structure and the at least one temperature sensor element. In principle, a variety of materials, such as polyimide, parylene, other plastic layers, metal oxides, silicon oxide, silicon nitride, silicon carbide, glass, etc. can be used as a dielectric layer.
An advantageous embodiment of the sensor proposes that the first dielectric layer has a lower thermal conductivity than the metal substrate. In this way, thermal coupling can be achieved between the dielectric layer and the heating structure formed on the first dielectric layer, which in turn leads to better thermal coupling to the medium and the overlying layers.
Another advantageous embodiment of the sensor proposes that the protective layer is formed either as a second dielectric layer or as a composite layer, wherein the composite layer comprises at least one second dielectric layer and a third, preferably dielectric layer. In particular, the composite layer is designed such that the second dielectric layer covers the at least one heating structure and the at least one temperature sensor element and that the third, preferably dielectric layer at least partially covers the second dielectric layer. The essentially flat layer of the at least one heating structure and the at least one temperature sensor element allows particularly cost-effective implementation of the sensors and use of the standard processes, which are being integrated in semiconductor technology and have already been established in micromechanics.
Alternatively, the composite layer is designed such that the second dielectric layer covers at least the lateral regions of the at least one heating structure and the surfaces of the at least one heating structure and the at least one temperature sensor element are covered by the third, preferably dielectric layer. Particularly preferred is the fact that the thermal conductivity of the third, preferably dielectric layer is better than that of the first and/or second dielectric layer. The selective layer structure allows a particularly good heat transfer between the heating structure and the medium, and also between the temperature sensor element and the medium.
Another alternative embodiment proposes that the first dielectric layer has a plurality of first sections, and wherein the at least one heating structure and the at least one temperature sensor element are arranged on different sections of the plurality of first sections. In particular, the first dielectric layer is divided into a maximum of five first sections. By the arrangement of the at least one heating structure and the at least one temperature sensor element on different sections, they advantageously allow for even better thermal insulation and thus, a particularly favorable response behavior of the sensor can be achieved.
Another advantageous embodiment proposes that the protective layer has a plurality of second sections. In particular, the number of the second sections of the protective layer corresponds to the number of first sections of the first dielectric layer.
According to another embodiment of the sensor according to the invention, the coefficient of thermal expansion of the first dielectric layer is selected such that a tensile stress acts on the membrane. By the tensile stress acting on the membrane, buckling of the membrane can be prevented, which in turn ensures that a very smooth membrane surface is formed.
According to another embodiment of the sensor according to the invention, the coefficient of thermal expansion of the second dielectric layer is in the range of the coefficient of thermal expansion of the first dielectric layer, in particular, the difference between the coefficients of thermal expansion is less than 20%, preferably less than 10%.
A last embodiment of the sensor according to the invention proposes that the first dielectric layer and the third, preferably dielectric layer comprise polyimide, Kapton and/or parylene, and/or the at least one heating structure and the at least one temperature sensor element comprise platinum or nickel or a nickel compound.
With regard to the method, the object is achieved by one used to manufacture a sensor as it is described in at least one of the preceding embodiments, wherein the sensor is held together in a composite comprising a plurality of sensors during the manufacturing process, for which the metallic substrate of the sensors is connected with the metallic substrate of the neighboring sensor using at least one bar, and wherein the at least one bar is destroyed to separate the sensor and remove it from the composite.
An advantageous embodiment of the method proposes that the at least one path is manufactured by an etching process using a masking layer.